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Linear Variable Differential Transformer (LVDT) Transducer |
An LVDT
Displacement Transducer comprises 3 coils; a primary and two secondary.
The transfer of current between the primary and the secondary of the
LVDT displacement transducer is controlled by the position of a magnetic
core called an armature.
On our position measurement LVDTs, the two transducer secondary are
connected in opposition.
At the centre of the position measurement stroke, the two secondary
voltages of the displacement transducer are equal but because they are
connected in opposition the resulting output from the sensor is zero.
As the LVDTs armature moves away from centre, the result is an increase
in one of the position sensor secondary and a decrease in the other.
This results in an output from the measurement sensor.
With LVDTs, the phase of the output (compared with the excitation phase)
enables the electronics to know which half of the coil the armature is
in.
The strength of the LVDT sensor's principle is that there is no
electrical contact across the transducer position sensing element which
for the user of the sensor means clean data, infinite resolution and a
very long life.
Our range of signal conditioning electronics for LVDTs handles all of
the above so that you get an output of voltage, current or serial data
proportional to the measurement position of the displacement transducer.
THE LINEAR
VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
The Linear Variable Differential Transformer (LVDT) is a displacement
measuring instrument and is not a strain-based sensor.
The LVDT models closely the ideal Zero th-order displacement sensor
structure at low frequency, where the output is a direct and linear
function of the input.
The LVDT is a variable-reluctance device, where a primary center coil
establishes a magnetic flux that is coupled through a mobile armature to
a symmetrically-wound secondary coil on either side of the primary.
Two components comprise the LVDT: the mobile armature and the outer
transformer windings. The secondary coils are series-opposed; wound in
series but in opposite directions.
When the moving armature is centered between the two series-opposed
secondary, equal magnetic flux couples into both secondary and the
voltage induced in one half of the secondary winding is balanced and 180
degrees out-of-phase with, the voltage induced in the other half of the
secondary winding.
The balanced condition provides total cancellation of secondary voltages
and therefore zero voltage output. When the moveable armature is
displaced from the balanced condition, more magnetic flux will couple
into one half of the secondary than into the other producing an
imbalance voltage output at the primary coil excitation frequency. The
output voltage of the LVDT is therefore a direct function of the
displacement of the mobile magnetic armature. The LVDT is, by
definition, a transformer and requires an oscillating primary coil
input.
The DC LVDT is provided with onboard oscillator, carrier amplifier, and
demodulator circuitry. The AC LVDT requires these components externally.
Due to the presence of internal circuitry, the DC LVDT is temperature
limited operating from typically -40 C to +120 C.
The AC LVDT is able to tolerate the extreme variations in operating
temperature that the internal circuitry of the DC LVDT could not
tolerate. Typically, LVDT’s will be excited by a primary carrier voltage
oscillating at between 50 hertz and 25 Kilohertz with 2.5 Kilohertz as a
nominal value. The carrier frequency is generally selected to be at
least 10 times greater than the highest expected frequency of the core
motion.
The external housing of the LVDT is fabricated of material having a
high-magnetic permeability therefore desensitizing the device from the
effects of external magnetic fields.
No sensing spring element exists within an LVDT and therefore, the
output of the sensor is hysteresis-free. Some LVDT displacement
measuring sensors are, however, provided with internal armature return
springs to allow profile measurement. When there exists no direct
contact with the moving armature is allowed no mechanical wear results.
The provision of linear bearings to prevent armature to coil structure
contact and to limit wear can greatly extend LVDT operating life
expectancies.
The strong relationship between core position and output voltage yields
a sensor design that shows excellent resolution, limited more by the
associated circuitry than the sensing method.
The internal core of the LVDT is generally constructed of an annealed
nickel iron alloy with the high-temperature limitations of the device
limited to the curie point of the core and the winding insulations used.
The thermal response characteristics of the LVDT are excellent for
static and quasi-static thermal environments due to the physical and
electrical symmetry of these devices. The physical symmetry also
contributes to excellent zero repeatability over time and temperature.
Most thermal-sensitivity shift errors result from the significant
thermal coefficient of resistance (TCR) of the copper transformer
windings. With increasing temperature, the primary coil resistance will
increase causing a decrease of the primary current in the
constant-voltage-excited case and therefore decreasing the magnetic flux
generated and voltage output correspondingly.
The use of constant-current excitation will ensure a constant primary
flux regardless of the coil resistance. Since the equivalent circuit of
the constant-current source is a voltage source with an infinite series
resistance, the use of a low-TCR resistance, in series with the primary,
will function in much the same manner as the piezoresistive
span-compensation resistor by causing the primary voltage to increase as
a function of temperature thus offsetting the TCR-induced losses. The
use of the series low-TCR resistor in the primary circuit allows the
constant-voltage source to appear to the LVDT as a constant-current
source.
Other thermally-active methods may also be used to compensate for the
primary winding TCR by causing the primary voltage to increase, with
rising temperature, in proportion to the increase in the primary coil
resistance. The temperature coefficient of magnetic permeability is
another contributor to the thermal-sensitivity shift and is compensated
out as a net effect by the means described above. Within approximately 2
seconds of power application the LVDT oscillator and demodulator
circuitry will stabilize sufficiently for dynamic measurement.
Due to self-heating of the primary coil, warm-up times for high
precision static measurement are comparable to strain gauged sensors and
are dependent upon the thermal stability of the measuring environment.
Summary:
LVDT operates
on the inductance ratio principle, Three coils are wired onto an
insulating tube containing an iron core, which is positioned within the
tube by the pressure sensor, Alternating current is applied to the
primary coil in the center, and if the core also is centered, equal
voltages will be induced in the secondary coils, Because the coils are
wired in series, this condition will result in a zero output, As the
process pressure changes and the core moves, the differential in the
voltages induced in the secondary coils is proportional to the pressure
causing the movement
LVDT type
pressure are available with ranges 0 - 30 psig to 0 - 10000 psig, It can
detect absolute, gauge or differential pressures
Advantages - Rugged; will not be easily damaged, Do not need to
compensate for friction; movable core not in touch
Disadvantage - Susceptible to mechanical wear and sensitivity to
vibration and magnetic
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